16th May 2025

A 44-year-old woman presents with 3 months of polyuria, polydipsia, weakness, and frequent waking during the night to urinate.

A 44-year-old woman presents with 3 months of polyuria, polydipsia, weakness, and frequent waking during the night to urinate. Her past medical history includes for bipolar disorder, and she is taking lithium citrate. Her vital signs are as follows:

• Blood pressure 110/80 mmHg
• Heart rate 80 bpm
• Respiratory rate 16 breaths/min, 
• Temperature 36.7 C

On examination she has dry mucus membranes. Paired osmolalities are performed which show plasma osmolality 290 mOsmol/kg, and urine osmolality 300 mOsmol/kg. A water deprivation test results in a serum osmolarity of 310 mOsmol/kg and a urine osmolality of 300 mOsmol/kg. After the administration of desmopressin, urine osmolality remains 300 mOsmol/kg. What is the next best step in management?

Select the SINGLE most appropriate option from the list below. Select ONE option only.